cmd/geth: add --config file flag (#13875)

* p2p/discover, p2p/discv5: add marshaling methods to Node

* p2p/netutil: make Netlist decodable from TOML

* common/math: encode nil HexOrDecimal256 as 0x0

* cmd/geth: add --config file flag

* cmd/geth: add missing license header

* eth: prettify Config again, fix tests

* eth: use gasprice.Config instead of duplicating its fields

* eth/gasprice: hide nil default from dumpconfig output

* cmd/geth: hide genesis block in dumpconfig output

* node: make tests compile

* console: fix tests

* cmd/geth: make TOML keys look exactly like Go struct fields

* p2p: use discovery by default

This makes the zero Config slightly more useful. It also fixes package
node tests because Node detects reuse of the datadir through the
NodeDatabase.

* cmd/geth: make ethstats URL settable through config file

* cmd/faucet: fix configuration

* cmd/geth: dedup attach tests

* eth: add comment for DefaultConfig

* eth: pass downloader.SyncMode in Config

This removes the FastSync, LightSync flags in favour of a more
general SyncMode flag.

* cmd/utils: remove jitvm flags

* cmd/utils: make mutually exclusive flag error prettier

It now reads:

   Fatal: flags --dev, --testnet can't be used at the same time

* p2p: fix typo

* node: add DefaultConfig, use it for geth

* mobile: add missing NoDiscovery option

* cmd/utils: drop MakeNode

This exposed a couple of places that needed to be updated to use
node.DefaultConfig.

* node: fix typo

* eth: make fast sync the default mode

* cmd/utils: remove IPCApiFlag (unused)

* node: remove default IPC path

Set it in the frontends instead.

* cmd/geth: add --syncmode

* cmd/utils: make --ipcdisable and --ipcpath mutually exclusive

* cmd/utils: don't enable WS, HTTP when setting addr

* cmd/utils: fix --identity

cmd/utils/customflags.go

@@ -17,6 +17,7 @@
 package utils
 
 import (
+	"encoding"
 	"errors"
 	"flag"
 	"fmt"
@@ -78,6 +79,58 @@ func (self DirectoryFlag) Apply(set *flag.FlagSet) {
 	})
 }
 
+type TextMarshaler interface {
+	encoding.TextMarshaler
+	encoding.TextUnmarshaler
+}
+
+// textMarshalerVal turns a TextMarshaler into a flag.Value
+type textMarshalerVal struct {
+	v TextMarshaler
+}
+
+func (v textMarshalerVal) String() string {
+	if v.v == nil {
+		return ""
+	}
+	text, _ := v.v.MarshalText()
+	return string(text)
+}
+
+func (v textMarshalerVal) Set(s string) error {
+	return v.v.UnmarshalText([]byte(s))
+}
+
+// TextMarshalerFlag wraps a TextMarshaler value.
+type TextMarshalerFlag struct {
+	Name  string
+	Value TextMarshaler
+	Usage string
+}
+
+func (f TextMarshalerFlag) GetName() string {
+	return f.Name
+}
+
+func (f TextMarshalerFlag) String() string {
+	return fmt.Sprintf("%s \"%v\"\t%v", prefixedNames(f.Name), f.Value, f.Usage)
+}
+
+func (f TextMarshalerFlag) Apply(set *flag.FlagSet) {
+	eachName(f.Name, func(name string) {
+		set.Var(textMarshalerVal{f.Value}, f.Name, f.Usage)
+	})
+}
+
+// GlobalTextMarshaler returns the value of a TextMarshalerFlag from the global flag set.
+func GlobalTextMarshaler(ctx *cli.Context, name string) TextMarshaler {
+	val := ctx.GlobalGeneric(name)
+	if val == nil {
+		return nil
+	}
+	return val.(textMarshalerVal).v
+}
+
 // BigFlag is a command line flag that accepts 256 bit big integers in decimal or
 // hexadecimal syntax.
 type BigFlag struct {